According to Google:
“From Thursday 13 August 2015 to Monday 17 August 2015, errors occurred on a small proportion of Google Compute Engine persistent disks in the europe-west1-b zone. The affected disks sporadically returned I/O errors to their attached GCE instances, and also typically returned errors for management operations such as snapshot creation. In a very small fraction of cases (less than 0.000001% of PD space in europe-west1-b), there was permanent data loss.”
Google claim that 0.000001 percent of storage in its persistent disk system was affected. That doesn’t sound like a large amount of data – it is 10 kilobytes for every 1 terabyte. However, a data centre is likely to be storing many petabytes. Overall this could be gigabytes of data loss.